In practice transmissions of motor vehicles with shifting elements are known, which comprise on the one hand interlocking shifting elements and on the other hand friction shifting elements. In each engaged gear of such a transmission a first number of shifting elements of the transmission are closed or engaged and a second number of shifting elements of the transmission are open or disengaged, and when a gearshift is carried out from a currently engaged gear to a target gear, at least one shifting element is opened or disengaged and at least one other shifting element is closed or engaged.
When, to carry out a gearshift or shifting operation in the transmission, an interlocking shifting element has to be closed or engaged, the closing or interlocking of the interlocking shifting element has to take place within a defined rotational speed window around a target rotational speed difference in order to avoid damaging the interlocking shifting element. To ensure this it is already known from practice that to carry out a gearshift in which an interlocking shifting element has to be closed or engaged, the interlocking shifting element is synchronized by at least partially closing a friction shifting element, with redundancy in the transmission.
For example, by at least partially closing a friction shifting element of the transmission, on the one hand a transmission input of the transmission and on the other hand a transmission output of the transmission can be braked in order to synchronize the interlocking shifting element. However, such synchronization of an interlocking shifting element of the transmission with redundancy in the transmission is only permitted at relatively slow driving speeds of the motor vehicle, i.e. when the current driving speed of the motor vehicle is lower than a corresponding limit value.
Since the driving speed of the motor vehicle is determined by rotational speeds on the drive output side, the problem can arise that for example as the result of a blocked drive output, a low current driving speed is estimated even though the motor vehicle is actually driving at a high speed, and because of this a redundancy in the transmission would be permitted even though the driving speed was high. However, to avoid critical driving situations and possible personal injury this eventuality must be securely and reliably avoided. Until now this has only been possible to an inadequate extent.
From DE 10 2009 056 793 A1 a method for operating a transmission is already known, the transmission besides friction shifting elements also comprising an interlocking shifting element, wherein to reduce a rotational speed difference at the interlocking shifting element and to synchronize the same at least one friction shifting element of the transmission is closed.